Host-computing systems, such as personal computers, are often operated as nodes on a communications network, where each node is capable of receiving data from the network and transmitting data to the network. Data is transferred over a network in groups or segments, wherein the organization and segmentation of data are dictated by a network operating system protocol, and many different protocols exist. In fact, data segments that correspond to different protocols can co-exist on the same communications network. In order for a node to receive and transmit information packets, the node is equipped with a peripheral network interface device, which is responsible for transferring information between the communications network and the host system. For transmission, a processor unit in the host system constructs data or information packets in accordance with a network operating system protocol and passes them to the network peripheral. In reception, the processor unit retrieves and decodes packets received by the network peripheral. The processor unit performs many of its transmission and reception functions in response to instructions from an interrupt service routine associated with the network peripheral. When a received packet requires processing, an interrupt may be issued to the host system by the network peripheral. The interrupt has traditionally been issued after either all of the bytes in a packet or some fixed number of bytes in the packet have been received by the network peripheral.
Networks are typically operated as a series or stack of layers or levels, where each layer offers services to the layer immediately above. Many different layered network architectures are possible, where the number of layers, the function and content of each layer may be different for different networks. The international standards organization (ISO) has developed an open systems interconnection (OSI) model defining a seven layer protocol stack including an application layer (e.g., layer 7), a presentation layer, a session layer, a transport layer, a network layer, a data link layer, and a physical layer (e.g., layer 1), wherein control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. The user of a host system generally interacts with a software program running at the uppermost (e.g., application) layer and the signals are sent across the network at the lowest (e.g., physical) layer.
One popular network architecture is sometimes referred to as a TCP/IP stack, in which the application layer is one of FTP (file transfer protocol), HTTP (hyper text transfer protocol), or SSH (secure shell). In these networks, the transport layer protocol is typically implemented as transmission control protocol (TCP) or user datagram protocol (UDP), and the network layer employs protocols such as the internet protocol (IP), address resolution protocol (ARP), reverse address resolution protocol (RARP), or internet control message protocol (ICMP). The data link layer is generally divided into two sublayers, including a media access control (MAC) sublayer that controls how a computer on the network gains access to the data and permission to transmit it, as well as a logical link control (LLC) sublayer that controls frame synchronization, flow control and error checking. The physical layer conveys the data as a bit stream of electrical impulses, light signals, and/or radio signals through the network at the physical (e.g., electrical and mechanical) level. The physical layer implements Ethernet, RS232, asynchronous transfer mode (ATM), or other protocols with physical layer components, where Ethernet is a popular local area network (LAN) defined by IEEE 802.3.
One or more layers in a network protocol stack often provide tools for error detection, including checksumming, wherein the transmitted messages include a numerical checksum value typically computed according to the number of set bits in the message. The receiving network node verifies the checksum value by computing a checksum using the same algorithm as the sender, and comparing the result with the checksum data in the received message. If the values are different, the receiver can assume that an error has occurred during transmission across the network. In one example, the TCP and IP layers (e.g., layers 4 and 3, respectively) typically employ checksums for error detection in a network application.
Data may also be divided or segmented at one or more of the layers in a network protocol stack. For example, the TCP protocol provides for division of data received from the application layer into segments, where a header is attached to each segment. Segment headers contain sender and recipient ports, segment ordering information, and a checksum. Segmentation is employed, for example, where a lower layer restricts data messages to a size smaller than a message from an upper layer. In one example, a TCP frame may be as large as 64 kbytes, whereas an Ethernet network may only allow frames of a much smaller size at the physical layer. In this case, the TCP layer may segment a large TCP frame into smaller segmented frames to accommodate the size restrictions of the Ethernet.
One or more of the network protocol layers may employ security mechanisms such as encryption and authentication to prevent unauthorized systems or users from reading the data, and/or to ensure that the data is from an expected source. For instance, IP security (IPsec) standards have been adopted for the IP layer (e.g., layer 3 of the OSI model) to facilitate secure exchange of data, which has been widely used to implement virtual private networks (VPNs). IPsec supports two operating modes, including transport mode and tunnel mode. In transport mode, the sender encrypts the data payload portion of the IP message and the IP header is not encrypted, whereas in tunnel mode, both the header and the payload are encrypted. In the receiver system, the message is decrypted at the IP layer, wherein the sender and receiver systems share a public key through a security association (SA). Key sharing is typically accomplished via an internet security association and key management protocol (ISAKMP) that allows the receiver to obtain a public key and authenticate the sender using digital certificates.
One significant challenge facing nodes on a communications network is that of collisions, which occur when two or more nodes attempt to transmit data simultaneously. Generally, a collision requires that collided frames/data of each node involved be retransmitted. It is noted that even the retransmission is at risk of another collision with previously collided nodes and/or other nodes transmitting other information. Such collisions and retransmissions can negatively impact performance of communication networks.